1. Field of the Invention
The present invention relates to a shift controlling apparatus for a continuously variable transmission for controlling a speed ratio thereof according to operating conditions of a vehicle, particularly to the shift control apparatus for improving a running feeling when the vehicle is rapidly increased.
2. Description of the Related Art
There are belt-type and toroidal-type continuously variable transmissions for vehicles. The belt-type continuously variable transmission has a primary pulley on an input shaft, a secondary pulley on an output shaft and a power transmission element such as a belt provided so as to wind on the pulleys. Speed ratios change continuously by changing widths of the pulleys.
As a kind of shift control apparatuses for the continuously variable transmission like this, there is the shift control apparatus having a memory in which a basic shift characteristics map is stored for setting a target speed for the primary pulley based on parameters such as a throttle opening, a vehicle speed and an engine speed which represent operating conditions of a vehicle while referencing the basic shift characteristics map and implementing a follow-up control such that an actual primary pulley speed converges to the target speed. Then, the speed ratio is controlled continuously from low to overdrive. Consequently, while the vehicle runs at a constant speed, when a driver operates to accelerate the vehicle rapidly or a kickdown shift is brought about, conventionally, the transmission is controlled by shift characteristics set in advance via the throttle opening, the vehicle speed and the engine speed. Then, the engine speed is drastically increased to a target speed of the primary pulley while the speed ratio setting is increased to the low side, and thereafter the speed ratio is gradually lowered. Accordingly, the engine speed increases before an increase in a vehicle speed at the time of rapid acceleration, and the vehicle speed increases thereafter, this sometimes gives the driver an uneasy feeling. In particular, in many cases ordinary continuously variable transmissions are provided with a shift characteristic in which the engine speed is held as low as possible while the vehicle is running at a constant speed to have an increased fuel economy, this serving to increase an uncomfortable feeling by the driver when the engine speed is increased from low to high by a rapid acceleration.
In contrast, in case a shift control is implemented in which the target speed for the primary pulley is set to increase in accordance with the increase in the engine speed, uncomfortable feeling at the time of a rapid acceleration can be eliminated, but this results in use of higher engine speed than required engine speed while the vehicle is running at a constant speed, and hence it becomes difficult to use an engine speed range which can provide a good fuel economy. On the other hand, JP-A-5-332426 discloses the continuously variable transmission in which the vehicle speed increases while repeating gradual increase and rapid decrease in the engine speed at the time of the rapid acceleration.